High energy rechargeable batteries have enabled an explosion in small handheld electronic devices such as cellular telephones, PDAs, and two way messaging devices. In these devices, the charging circuit for the rechargeable battery is typically located outside of the device in order to avoid penalizing the device by the size, weight and heat generated by the charging circuit. A cable from the charging circuit will typically plug into a connector on the portable device. Typically a pair of back-to-back-FETs are utilized to permit the battery to be charged, but avoid the possibility of discharging the battery when charger is inactive. An inactive charger may act as a load on the battery. These FETs must be able to withstand the surges associated with battery charging and therefore are relatively large. Eliminating one of these two FETs would reduce the “real estate” utilized by the circuit, and thus reduce the cost of the integrated circuit within the handheld device. If the second FET is omitted, the battery charger pin will rise almost to the battery voltage through the back gate diode of the remaining FET. It is necessary to monitor the charger pin to determine when to keep all circuits off and conserve power, and when to turn on the charging circuitry to replenish the battery.
A circuit to perform this function is shown in FIG. 1A, generally as 100. The circuit comprises a comparator 102 having its non-inverting input coupled to the pin receiving battery voltage VCHGR through a resistance RTH. The inverting input to the comparator is coupled to the pin receiving the battery voltage VBATT. The back gate diode of the FET is shown as 106. In order to establish a threshold voltage which is slightly above the battery the battery voltage, a current IREF is drawn from node 104 by reference generator 108.
FIG. 1B shows the output charger detect voltage (CH_DET) as a function of the input voltage VCHRG. As can be seen from FIG. 1B, once the input voltage rises the threshold voltage above the battery, the output CH_DET from the comparator will change state, and that signal can be utilized to allow the battery charger to charge the internal rechargeable battery in the handheld device. A weakness of the circuit shown in FIG. 1A is that in order to have an accurate threshold voltage which is temperature compensated, the current drawn by the comparator circuit and the reference generator circuit is considerable for a battery powered device. This circuit runs continuously whether or not the handheld device is on. The power consumption of this circuit is critical, because it can noticeably change the usable lifetime of the battery between charges.